Underwater search and recovery

Underwater search and recovery is the process of locating and recovering underwater objects, often by divers, but also by the use of submersibles, remotely operated vehicles and electronic equipment on surface vessels.
Most underwater search and recovery is done by professional divers as part of commercial marine salvage operations, military operations, emergency services, or law enforcement activities.
Minor aspects of search and recovery are also considered within the scope of recreational diving.
Professional search and recovery
The scope of professional search and recovery includes the following applications:
Marine archeology - search for artifacts of historical interest and importance, and where applicable the recovery of such artifacts for study.
Marine salvage - search for lost material of value and recovery thereof
Environmental protection - search for environmentally undesirable material and disposal thereof
Forensic S&R - search for and recovery of material relevant to police investigations
Emergency S&R - Search for and recovery of victims of accidents and disasters
Military S&R - Search and recovery related to military operations and material
Scientific S&R - search for objects of scientific interest, recovery of instrumentation and samples
Recreational search and recovery
Search and recovery diving is also frequently undertaken as part of recreational diving, and most diver training organisations have dedicated training courses on the subject. Search and recovery is generally considered a more hazardous speciality diving course.
Underwater search and recovery used to form a mandatory component of the Advanced Open Water Diver training course for many North American diver training agencies, which, in addition to learning basic search and recover skills, also assisted in teaching students to cope with task loading.
Although the scale, value and equipment used in commercial and recreational search and recovery are enormously different, the basic premise remains the same in each case.
Search
Diver search
Underwater searches, much like above water searches, are designed around specific search patterns. The most common forms of underwater search patterns are:
Circular search
Jackstay search
Expanding square search
"U" pattern search
The patterns are usually performed by divers in pairs or teams below the water, but they can also be conducted by use of a tender who may be a snorkeller at the surface, a person on a towing boat, or a person located on the shore.
In simple search operations, the patterns will usually be conducted by the divers simply looking visually for the object. In more sophisticated search operations, underwater magnetometers or hand held sonar may be used.
Types
Diver training traditionally divides searches into two categories, specific and non-specific.
A specific search is an attempt to locate a known object in a known area where it was believed to be lost even if the time period is undefined, and the search terminates upon the location of the object. The classic example of this would be an item lost overboard from a boat, which needs to be recovered.
A non-specific search is a search for either a type of object or anything valuable within the dive locale. The discovery of a relevant object does not usually terminate the search until the entire search area has been covered, or the search terminates early for other reasons (air supply, no decompression limits, etc.).
ROV searches
Searches using non-visual methods
A magnetometer is a device that measures magnetic field or magnetic dipole moment. Different types of magnetometers measure the direction, strength, or relative change of a magnetic field at a particular location. Magnetometers can be used as metal detectors: they can detect only magnetic (ferrous) metals, but can detect such metals at a much greater distance than conventional metal detectors which rely on conductivity; they are capable of detecting large ferrous objects over considerable distances.
Side-scan sonar
A dragline is a line which is towed ether across the bottom, or at a depth controlled by floats and weights, or hydrodynamically, by paravanes or otter-boards, that is intended to snag the target, indicating its position.
Cadaver dogs
Recovery
Recovery techniques depend upon the type and size of the object, and the depth at which it is found.
Smaller objects, such as a coin or camera, can simply be carried up by the diver. Training agencies vary in what they specify to be the maximum weight that can be safely carried up unassisted by a scuba diver, but normally the limit is set around . Anything heavier represents a material change to the diver's buoyancy control, and may put the diver at risk from an uncontrolled ascent if contact with the object is lost during ascent. Professional surface supplied divers working from a bell or stage can carry heavier objects to the stage, but they usually have an option to use a lifting line independent of the diver transport platform. Similarly, small objects can be recovered directly by lifting by an ROV or crewed submersible with manipulator capability.
Medium-sized objects are normally recovered using a lifting bag, and professional divers are trained in lifting bag techniques (it is usually considered specialty training in recreational diving). The most common hazard is entanglement with lines whilst filling the lifting bag from the diver's alternate air source. This risk, when coupled with the possibility of rapid ascent following the freeing of the object from suction caused by it resting upon the sea bed, can seriously compromise the safety of the recovery team if done improperly. Lift bags can be rated up to several tons, but these are beyond the capacity of most recreational divers, and there is no recreational diver training for their use. A cubic metre of air per tonne at ambient hydrostatic pressure is needed. This is not practicable for cylinders carried by the diver (the most common scuba cylinder size has a total capacity of at surface pressure). Large lifting bags are normally filled from a low-pressure compressed air hose from the surface, which may be attached directly to the lifting bag.
Significantly larger objects usually require specialised commercial lifting equipment, either a winch attached to a boat or platform, or specialised equipment to seal and inflate sunken vessels.
See also
References
Footnotes
Underwater diving procedures
Emergency services